The present invention relates in general to oscilloscopes and in particular to a system for determining input signal magnitudes represented by points on a waveform displayed on an oscilloscope screen.
Oscilloscopes typically display a waveform representing an input signal on a screen, the waveform comprising a graph of input signal magnitude as a function of time. The graph is created by an electron beam moving across the screen, the vertical position of the beam being controlled by the input signal and the horizontal positoin of the beam being controlled by a voltage ramp (sweep) signal. A grid having vertical and horizontal lines is often superimposed on the screen to permit an operator to gauge vertical and horizontal displacement of any selected point on the waveform from a reference point on the grid. Given the relationship between vertical grid divisions and voltage magnitude (the vertical gain of the oscilloscope in volts/grid division) and the relationship between horizontal grid divisions and time (the sweep speed of the beam in grid divisions/second), the operator can determine the relative magnitude or timing of the selected point along the waveform with respect to the reference point by measuring the vertical or horizontal displacement of the selected point in grid divisions and multiplying by the vertical gain or sweep rate. Oscilloscope operators may be interested, for example, in determining the peak values of periodic signals, but sometimes find it difficult to see the signal peak, particularly when the input signal contains high frequency components which are of such short duration that they are only faintly visible on the screen, or not visible at all. Even when the peak can be observed, an operator normally cannot gauge the vertical position of the peak within the grid with high accuracy.
In addition, "vertical" amplifier systems in an oscilloscope, which amplify the input signal in order to produce a signal for controlling the vertical position of the beam, exhibit gain and offset errors which vary with oscilloscope operating temperature, component aging and other factors. These gain and offset errors cause the actual vertical gain of the oscilloscope to differ from its nominal value and cause the actual location of a reference point on the grid to vary from its nominal, expected location. Therefore the product of the nominal gain and the vertical displacement of a selected waveform point from the nominal reference point location does not accurately reflect the actual magnitude represented by the waveform point.
What is needed is a system for accurately determining signal magnitudes represented by points on waveforms displayed on an oscilloscope screen, and for communicating the measured magnitudes to an oscilloscope operator.